This function adds the cke_disabled class to this.getElement() which corresponds to the a tag for the button. Therefore the a tag can be styled using this class when the button is disabled.
However, the cke_disabled class is not added to the span tag which displays the label for the button (see the attached screen capture). Therefore, there is no way to style the text for disabled buttons.

The disable function should be updated to also add the cke_disabled class to the span tag. Perhaps doing something like this: